understanding the simple deform modifier (TAPER - FACTOR)
If you look at the source code of the simple deform modifier (1), the Taper transformation is done as follows. I assume that you selected the Z-axis as the deform axis.

scale = z * factor;
new_x = x + x * scale;
new_y = y + y * scale;
new_z = z;


So, every vertex of the object is scaled for the X and Y axis (unless, you have them locked). The z-coordinate remains untouched; maybe also the Y-coordinate, if you have locked it. You were right that the factor is mixed with other variables. As you can see, the factor is multiplied by the z coordinate. The further away a vertex is from the origin (as you suspected), the larger the scaling factor. At origin, z = 0, so the scaling is 0. At the top, z=1 (I assume!), so the scaling = factor. All vertices between origin and top are scaled between 0 and factor.

In the attached picture, I applied the taper modifier only to the group of top-right vertices. The origin of the cube is at the bottom. The factor is -1.000.

If the factor is -1.5, then

new_x = 1 + 1 * (1 * -1.5)= -0.5;


If you want the top-right vertices to collapse with the top-left vertices, the factor should be - 2; because

new_x = 1 + 1 * (1 * -2)= -1; 

which is the x-location of the top-left vertices.